"it's a genuinely really lovely thought, but one of those 'if we were in a parallel universe' things I'm afraid. I have a strict dating-never-mixes-with-work policy (rather personal reasons) and can't break it even for you! Happy to go for a drink as friends, but no worries if you'd rather just stick to business as we have been doing"

Obviously leave out the drink as friends bit if you don't actually want to. Not sure if that's a bit cop out or not. It's not meant to be, I was going for not a vibe of non-offensive non-embarrassing.

Wow so sorry Op you have already turned him down as the Pp says, I don't know why i didn't see that line. Ignore my silly advice.

How did you turn him down? I bet it won't be as bad as you think if he's a decent enough bloke... I always find the awkwardness of these things are worse in my imagination but totally understand your concerns.